1. Field of the Invention
The present invention relates to an image forming apparatus and image forming method of generating a desired image formation pattern by converting an input image signal.
2. Description of the Related Art
Generally, an electrophotographic system, particularly a color copying machine using color toners of a plurality of colors has a look-up table for converting an image signal into a signal value meeting the characteristics of the engine used, in order to obtain desired density gradation characteristics. A color copying machine has such look-up table for each of yellow, magenta, cyan, and black and can output a desired full-color image by individually optimizing these colors.
An electrophotographic image forming apparatus, however, readily changes its characteristics in accordance with, e.g., the surrounding environment and the use state, so it is difficult to constantly output images stable in the tone of color by using fixed look-up tables. Accordingly, conventional apparatuses of this sort include a development density detecting means for detecting the density of a developed image formed on a photosensitive drum or the like. On the basis of information of the density detected by this development density detecting means, look-up tables are newly formed or corrected so that desired gradation characteristics can be obtained.
A conventional image forming apparatus having this development density detecting means will be described below. FIG. 9 shows the overall arrangement of a conventional electrophotographic digital copying machine.
In this conventional digital copying machine shown in FIG. 9, a CCD 1 reads an image of an original 21, and an amplifier 2 amplifies the obtained analog image signal to a predetermined level. After that, an analog/digital converter (A/D converter) 3 converts the amplified signal into a digital image signal of, e.g., 8 bits (0 to 255 gray levels).
This digital image signal is supplied to a .gamma. converter (e.g., a converter which is constructed of 256-byte data and converts density in a look-up table manner) 5 where the signal is .gamma.-corrected. This digital image signal is again converted into an analog image signal and supplied to one input terminal of a comparator 11.
A triangular-wave signal of predetermined period generated by a triangular-wave generator 10 is supplied to the other input terminal of the comparator 11. The analog image signal supplied to one of the input terminals of the comparator 11 is compared with the triangular-wave signal of predetermined period generated by the triangular-wave generator 10, and is modulated in pulse width. This pulse-wave-modulated binary image signal is input to a laser driver 12 and used as an emission ON/OFF control signal of a laser diode 13.
A laser beam emitted from the laser diode 13 is scanned in a main scan direction by a well-known polygonal mirror 14 and irradiates a photosensitive drum 17, which is an image carrier rotating in an arrow direction, via an f.theta. lens 15 and a reflecting mirror 16, forming an electrostatic latent image.
This photosensitive drum 17 is evenly charged-removed by an exposing unit 18 and evenly charged, e.g., negatively, by a primary charger 19. After that, the photosensitive drum 17 is irradiated with the aforementioned laser beam to form an electrostatic latent image corresponding to the image signal. A developing unit 20 develops this electrostatic latent image into a visual image (toner image).
During the development, an AC bias component is superposed on the developing unit to improve the DC bias component and development efficiency meeting the electrostatic latent image formation conditions. This toner image is transferred, by the action of a transfer charger 22, on to a transfer medium 23 held on a transfer medium carrier belt 27 that stretches between two rollers 25 and 26 and is endlessly driven in an arrow direction.
The residual toner on the photosensitive drum 17 is scraped off and collected by a cleaner 24 later. For the sake of simplicity, FIG. 9 shows only a single image forming station (including the photosensitive drum 17, the exposing unit 18, the primary charger 19, the developing unit 20, and the like). In the case of a color image forming apparatus, however, image forming stations corresponding to yellow, magenta, cyan, and black are arranged above the transfer medium carrier belt 27 along its moving direction. Alternatively, developing units of yellow, magenta, cyan, and black are placed in a rotatable housing, and a desired developing unit is opposed to the photosensitive drum 17 to develop a desired color.
When the development by the developing unit 20 progresses, the toner amount in the developing unit 20 reduces, so no desired density can be secured any longer. Therefore, a video counter 4 is used to extract by analogy the use amount of toner from the image formation pattern.
A CPU 6 stores the toner use amount extracted by the video counter 4 into a RAM 6a. In the case of a color image forming apparatus, the CPU 6 adds up the use amounts of toner of each color. When a predetermined use amount is reached, the CPU 6 activates a toner supply driver 7 and rotates a toner supply motor 28 to drive a toner supply mechanism 30, thereby supplying toner 29 in a toner cartridge 8 into a predetermined developing unit 20. Reference numeral 21 denotes toner supplied into the developing unit.
Not only for copying machines, several halftoning methods (image processing patterns) of gradation reproduction methods can be used. So-called multi-valued reproduction reproduces gradation in units of dots by using a triangular wave described previously. So-called binary reproduction reproduces gradation by forming a plurality of dot matrices at two values: a dot is formed and a dot is not formed. In another method, a matrix is formed by multiple values. An optimum one is chosen from these methods in accordance with the type (e.g., a printed original, a character original, or an image signal from a controller) of input image.
In any method, desired gradation characteristics can be secured only when the toner supply amount in the developing unit 20 is controlled to a desired amount. To control this toner supply amount, the conventional approach is to detect the density of a patch-like toner image (to be referred to as a "gradation control patch" hereinafter) obtained by developing an electrostatic latent image formed by a gradation control image signal. That is, this gradation control patch is irradiated with light from a light source such as an LED, the reflected light is received by a built-in photoelectric element, and the output value is converted into density. In accordance with information of the detected density signal, look-up tables are newly formed or corrected. In this manner, the toner supply amount in the developing unit 20 is controlled to maintain desired gradation characteristics.
Some conventional apparatuses, however, obtain gradation characteristics by using various image processing patterns for one printer engine. If this is the case, it is necessary to form gradation control patches for the individual image processing patterns, measure the respective density characteristics, and form look-up tables and the like on the basis of the measurement results. This is very time-consuming and inconvenient.